The present disclosure relates to the field of display technologies, and more particularly to an organic light emitting diode (OLED) pixel driving circuit and an OLED display apparatus.
In recentyears, display technologies of organic light emitting diode (OLED) develop rapidly to promote the curve and flexible touch display products to market quickly, and the technologies of relevant field are innovated from day to day. OLED is a diode capable of emitting light by utilizing organic semiconductor material and electroluminescent material for injecting and incorporating charge carriers under the excitation of electrical field. OLED display apparatus has the advantages of self-illuminating, low driving voltage, high illuminating efficiency, short response time, high sharpness and contrast ratio, wide viewing angle with nearly 180 degrees, wide range of operation temperature, flexible display capability and big area display with full colors. OLED display apparatus is commonly considered as a kind of display apparatus with most development prospect.
OLED is a component which is driven by current. OLED illuminates while current flows across, and the illuminance depends on the current volume flowed across the OLED. Most existed integrated circuits (ICs) only transmit voltage signals; thus a pixel driving circuit is needed to implement a task of converting voltage signals into current signals. The conventional OLED pixel driving circuit usually uses a circuit scheme with two transistors and one capacitance (2T1C) to convert voltage signals into current signals.
Referring to FIG. 1, FIG. 1 is a schematic view of a conventional 2T1C pixel driving circuit for the OLED. As shown in FIG. 1, the conventional 2T1C pixel driving circuit for the OLED includes a first thin film transistor (TFT) T1, a second TFT T2 and a capacitor Cs, wherein the first TFT T1 is a driving TFT, the second TFT T2 is a switch TFT, the capacitor Cs is a storage capacitor. Specifically, a source of the first TFT T1 is electrically connected to an anode of an OLED D0, a drain of the first TFT T1 is connected to a power voltage OVDD, and a gate of the first TFT T1 is connected to a first node G. A cathode of the OLDE D0 is connected to a common ground OVSS. A source of the second TFT T2 is electrically connected to the first node G, a gate of the second TFT T2 connects with a scan signal Scan, and a drain of the second TFT T2 is connected to a data signal Data. The capacitor Cs has one end connecting with the gate of the first TFT T1, and another end electrically connecting with the source of the first TFT T1. While the OLED is displaying, the scan signal controls the second TFT T2 in on state, and the data signal is transmitted through the second TFT T2 into the gate of the first TFT T1 and the capacitor Cs, and then the second TFT T2 is controlled in off state, the voltage of gate of the first TFT T1 is still with the data signal voltage because charges are stored in the capacitor Cs. Subsequently, the first TFT is controlled in on state, a driving current flows through the OLED D0 to drive the OLED D0 to illuminate.
The I-V equation of transistor:Ids,sat=k·(VGS−Vth,T1)2=k·(VG−VS−Vth,T1)2 
in the equation, k is an intrinsic conductivity factor value, Ids,sat value is related to a voltage threshold of the driving TFT, that is, the TFT T1.
As a result of the uncontrollable factors during the manufacturing processes of the panel, the voltage threshold Vth of the driving TFT of each sub-pixel in the panel may different. Consequently, the currents flowed into each OLED may not be identical, even the voltage Vdata of data signal Data is input to each driving TFT of sub-pixel uniformly. This is the reason that the uniformity of image display quality is difficult to realize.
Additionally, as the accumulation of driving time of the driving TFT, the material of TFT may age and degenerate; this makes the voltage threshold Vth of the driving TFT drift. Moreover, as the aged conditions of material of each TFT may different, this results in the different drift of voltage threshold Vth of each driving TFT and also results in the non-uniform display quality of panel. Further, the aged condition of TFT material becomes more serious as the continuous accumulation of driving time of the driving TFT. The currents flowed across each OLED may not be identical, even the driving voltage is same; this causes the non-uniform illuminance of OLED. Moreover, the start voltage of OLED will increase due to the aging of OLED, and the current flowed into OLED will get smaller gradually; this brings the problems of degrading illuminance and illuminating efficiency of panel.
Referring to FIG. 2, FIG. 2 is a schematic view of a conventional 3T1C (three transistors, one capacitor) pixel driving circuit for the OLED. As shown in FIG. 2, the conventional 3T1C pixel driving circuit for the OLED includes a first TFT T1, a second TFT T2, a third TFT T3, a capacitor CLED and a capacitor Cp, wherein the first TFT T1 is a driving TFT. Comparing with the 2T1C pixel driving circuit shown in FIG. 1, the conventional 3T1C pixel driving circuit supplements following components: the CLED electrically connected between the anode and the cathode of LED D0; the third TFT T3 has a source electrically connected to a source of the first TFT T1, a drain connected to a sensing control signal Sense and a gate connected to a scan signal Scan; a capacitor Cp has one end connected to the drain of the third TFT T3 and another end connected to a common ground; a reference voltage Vref connects with the sensing control signal Sense; and an analog to digital converter ADC0 connects with the sensing control signal Sense through a switch SW0. The conventional 3T1C pixel driving circuit compensates the voltage threshold Vth of the driving TFT T1 by detecting the voltage threshold Vth of the driving TFT T1; however, such design is only capable of compensating the voltage threshold Vth, but unable to compensate the effect of aging for the OLED. The k value will drift when the OLED is aged, and the aged conditions of material of each TFT may differ; this results in the problems for degradation of illuminance, reduction of illuminating efficiency and non-uniformity of illuminance of panel.
Therefore, the technical problems of compensating the drift of the voltage threshold Vth of the driving TFT and the drift of k value in the aged OLED, and improving the uniformity of illuminance of OLED panel are needed to be resolved.